Limitation of the Polarization by Radiation Trapping in a Helium Afterglow Electron Source

(Received 7 April 1994, revised 23 September 1994, accepted 28 September 1994)

Abstract
A polarized electron source using an optically pumped helium afterglow was built at Orsay. Unfortunately the spin polarization decreases at high metastable densities. Calculations of the radiation trapping effects in a weak magnetic field are presented using the Anderson formalism. Comparison with experimental data leads to the conclusion that these trapping effects are one explanation of this polarization decrease. Effects of the main parameters are studied. Some deductions for a new design can be made.